Method for fabricating broadband cholesteric liquid crystal film

ABSTRACT

The invention provides a broadband cholesteric liquid crystal film, a method for fabricating the same, a polarization device employing the same, and high light efficiency liquid crystal display employing the same. The cholesteric liquid crystal film is a single-layer liquid crystal material structure, and has a top surface and a bottom surface. Further, the cholesteric liquid crystal film includes a first region, a second region, and a third region, and the first region is adjacent to the top surface of the cholesteric liquid crystal film, the third region is adjacent to the bottom surface of the cholesteric liquid crystal film, and the second region is located between the first and third regions, and the average helical pitch P1 of the first region and the average helical pitch P3 of the third region are both larger than the average helical pitch P2 of the second region.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of pending U.S. patent application Ser.No. 13/284,163, filed on Oct. 28, 2011 and entitled “Broadbandcholesteric liquid crystal film, method for fabricating the same,polarization device, and high light efficiency liquid crystal displayemploying the same”, which is based upon and claims the benefit ofpriority from the prior Taiwan Patent Application No. 099146534, filedon Dec. 29, 2010, the entire contents of which are incorporated hereinby reference.

BACKGROUND

1. Technical Field

The disclosure relates to a cholesteric liquid crystal film and anarticle employing the same, and in particular relates to a broadbandcholesteric liquid crystal film and an article employing the same.

2. Description of the Related Art

Cholesteric liquid crystal film is quite a unique optical elementexhibits selective light reflection and circular dichronism. It iswidely applied in various display devices or photoelectric elements,such as polarizer films or backlight modules of LCDs.

Cholesteric liquid crystal owns a layer-by-layer molecular stackingstructure. The long axis directions of the molecular layers of thestacked molecular structure are in essence the same, Said moleculardirectors are twisted a small angle from one layer to another one layer.The helical pitch of a cholesteric liquid crystal is defined as thedistance between liquid crystal molecular layers which are located at anangle of 360 degrees to each other along the long axis direction. Thestacked molecular structure of a cholesteric liquid crystal layer can beof left-handed orientation or right-handed orientation depending on thechirality of the chiral compounds of the cholesteric liquid crystal.Specifically, a circular polarized light having the same circulardichronism as the cholesteric liquid crystal layer will be reflected,and the other circular polarized light having an opposite circulardichronism will pass through the cholesteric liquid crystal film.

According to the Bragg reflection rule, the relationship betweenwavelength (λ) of a cholesteric liquid crystal, an average refractionratio (n_(ave)) of a thin film, and a helical pitch (P) of a cholestericliquid crystal molecule is:

λ=n _(ave) ·P

The reflection bandwidth (Δλ) relates to the birefringence (Δn) andhelical pitch (P) of cholesteric liquid crystal molecules, according tothe relationship equation:

Δλ=Δn·P

According to this formula, the selective reflection bandwidth of commoncholesteric liquid crystal is strongly related to its birirefringence(Δn) and only about several tens of nanometers (such as 40-50 nm). It isonly part of range of visible light, which covers bandwidth from 400 to700 nm. It is thus important to improve the assembly structure ofcholesteric liquid crystal molecules to meet the requirements for awider selective reflection bandwidth.

Since the selective reflection bandwidth of a common single layer of acholesteric liquid crystal film is limited, a plurality of layers of acholesteric liquid crystal film are prepared by coating by multipletimes such that the bandwidth of each layer can be combined together soas to cover the whole wavelength range of visible light.

For example, U.S. Pat. No. 6,016,177 discloses a multilayer cholestericliquid crystal film 10 including layers 12, 14, and 16 of cholestericliquid crystal polymers having different helical pitches, wherein thelayers 12, 14, and 16 are combined by adhesive layers 18, as shown inFIG. 1. However, a plurality of manufacturing processes is required andthere are multiple alignment interferences at the interfaces whichadversely affect the polarizing effect thereof. Furthermore, themanufacturing process of the multilayer cholesteric liquid crystal film10 is complicated, and the yield is reduced thereof.

In order to solve the aforementioned problems, a method for fabricatinga multilayer cholesteric liquid crystal film via continuous coatings isprovided. Due to the absence of the adhesive layers, the multilayercholesteric liquid crystal film exhibits improved light transmittance.The layers of the multilayer cholesteric liquid crystal film, however,have similar surface tensions, which may cause coating defects withinthe multilayer cholesteric liquid crystal film.

On the other hand, since a single-layer cholesteric liquid crystal filmhas a simplified manufacturing process and less coating defectsresulting from continuous coatings, a single-layer cholesteric liquidcrystal film is desired to replace the conventional multilayercholesteric liquid crystal films.

U.S. Pat. No. 5,506,704 discloses a single-layer cholesteric liquidcrystal film. The variation of the helical pitch is achieved via along-term UV exposure. Thereby, the helical pitch in the thicknessdirection is varied progressively. However, a the long time UV exposureis needed will be problematic for mass production, especially for lowproduction efficiency, and specialized equipment needed.

U.S. Pat. No. 7,311,952 discloses a liquid crystalline film withbroadened reflection bandwidth fabricated via a two-stage polymerizationmethod. First, a coating of a polymerizable liquid-crystalline materialis partially polymerized via an actinic radiation in an environment,which has an inhibiting action on the polymerization (such as air or airenriched with oxygen), to form a semi-polymerized structure. After abriefly re-alignment period, the semi-polymerized film is fullypolymerized via a high-energy actinic radiation to form a soilied film.Referring to FIG. 2, the obtained cholesteric liquid crystal film 20 canbe derived into three regions according to the helical pitch. The region22 adjacent to the air top surface 21 of the cholesteric liquid crystalfilm 20 has the shortest helical pitch, the region 24 disposed betweenthe region 22 and the region 26 has the longest helical pitch, and theregion 26 has the middle value of the helical pitches in between theregion 22 and region 24 does. The method, however, has an inherentlyhigh degree of process uncertainty due to the additional alignment timeand harsh reaction conditions (such as radiation energy) of the partialpolymerization.

SUMMARY

In this article, a newly cholesteric liquid crystal (CLC) helical pitchis proposed, An exemplary embodiment of a broadband cholesteric liquidcrystal film is a single-layer liquid crystal material structure and hasa top surface and a bottom surface, wherein the broadband cholestericliquid crystal film consists of a first region, a second region, and athird region. The first region is adjacent to the top surface of thecholesteric liquid crystal film, the third region is adjacent to thebottom surface of the cholesteric liquid crystal film, and the secondregion is located between the first and third regions. Particularly, theaverage helical pitch P1 of the first region and the average helicalpitch P3 of the third region are both larger than the average helicalpitch P2 of the second region.

Further, this disclosure also provides a method for fabricating theaforementioned broadband cholesteric liquid crystal film, including thesteps (A)-(D). In step (A), a polymerizable cholesteric liquid crystalcomposition and a photoinitiator are dissolved in a solvent to prepare asolution for coating. In step (B), the solution is coated onto asubstrate (such as a pre-treated alignment substrate), and a cholestericliquid crystal (CLC) coating layer is obtained. In step (C), the CLCcoating layer is heated in a first atmosphere containing a firstpre-fixed oxygen concentration, wherein the first pre-fixed oxygenconcentration is a fixed value of between 10-30 vol %, preferably 15-25vol %. In step (D), the heated CLC layer is cured in a second atmospherecontaining a second pre-fixed oxygen concentration, obtaining theaforementioned broadband cholesteric liquid crystal film, wherein thesecond pre-fixed oxygen concentration is a fixed value of between0.01-10 vol %.

Moreover, the disclosure also provides a polarization device, includingthe aforementioned broadband cholesteric liquid crystal film; and aretardation film disposed on the broadband cholesteric liquid crystalfilm.

The retardation film has an in-plane retardation value (Ro)substantially one-quarter of the wavelength of visible light, and anout-of-plane retardation (Rth), wherein the ratio (Rth/Ro) is between0.5-3

According to some embodiments of this disclosure, a high lightefficiency liquid crystal display is provided. The high light efficiencyliquid crystal display includes a top substrate and a bottom substrate,a first electrode formed on a bottom surface of the top substrate, and asecond electrode formed on the top surface of the bottom substrate,wherein a liquid crystal cell is sandwiched between the top substrateand the bottom substrate, and the aforementioned broadband cholestericliquid crystal film disposed inside or outside of the liquid crystalcell.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 shows a cross section of a conventional multilayer cholestericliquid crystal film with adhesive layers.

FIG. 2 shows a cross section of a conventional single-layer cholestericliquid crystal film.

FIG. 3 shows a cross section of a broadband cholesteric liquid crystalfilm according to an embodiment of the invention.

FIG. 4 shows a scanning electron microscope (SEM) photograph of thebroadband cholesteric liquid crystal film (A) of Example 1.

FIG. 5 shows a cross section of a Polarization device according to anembodiment of the invention.

DETAILED DESCRIPTION

The disclosure provides a cholesteric liquid crystal film with a broadreflection bandwidth which is a single-layer liquid crystal materialstructure and has several regions respectively having different averagehelical pitches. Particularly, the helical pitches of the severalregions have a predetermined magnitude relationship. Further, thebroadband cholesteric liquid crystal film of this disclosure can befurther combined with a retardation film to constitute a reflection typepolarization device, with high contrast, and high transmittance, whichis suitable to be applied in polarizers, brightness enhancement devices,or 3D Image display devices.

Broadband cholesteric liquid crystal film

The term “broadband cholesteric liquid crystal film” means a cholestericliquid crystal film with a broad selective reflection bandwidth. Thebroadband cholesteric liquid crystal film of this disclosure has asingle-layer cholesteric liquid crystal material coating layer. The term“single-layer cholesteric liquid crystal material” means a cholestericliquid crystal film prepared by curing a single cholesteric liquidcrystal composition (including the composition made of nematic liquidcrystal and chiral compound) in the presence of a photoinitiator. Thebroadband cholesteric liquid crystal film of the disclosure is asingle-layer film, thereby eliminating complicated manufacturingprocesses and coating defects caused when forming a multilayercholesteric liquid crystal film.

The broadband cholesteric liquid crystal film of this disclosure has aselective reflection bandwidth includes most of the wavelength range ofvisible light. Further, the said reflection bandwidth is larger than 150nm, such as 150-350 nm. In FIG. 3, wherein the cholesteric liquidcrystal film 100 has a top surface 101 and a bottom surface 103, and thebottom surface 103 contacts with a substrate 50. The cholesteric liquidcrystal film 100 consists of a first region 102, a second region 104,and a third region 106, wherein the first region 102 is adjacent to thetop surface 101 of the cholesteric liquid crystal film 100, the thirdregion 106 is adjacent to the bottom surface 103 of the cholestericliquid crystal film 100, and the second region 104 is in between thefirst region 102 and the third region 106. It should be noted that thefirst region 102, the second region 104, and the third region 106 of thecholesteric liquid crystal film 100 have different average helicalpitches. The first region 102 had an average helical pitch P1, thesecond region had an average helical pitch P2, and the third region hadan average helical pitch P3. Particularly, the average helical pitch P1of the first region 102 and the average helical pitch P3 of the thirdregion 106 are both larger than the average helical pitch P2 of thesecond region 104. In an embodiment of this disclosure, the averagehelical pitch P1 of the first region, the average helical pitch P2 ofthe second region, and the average helical pitch P3 of the third regionare defined by the following equation: P1>P3>P2. Namely, the cholestericliquid crystal film 100 achieves an approximately symmetrical helicalpitch distribution from the top surface 101 to the center and from thebottom surface 103 to the center of the cholesteric liquid crystal film100.

According to another embodiment of the disclosure, a part of thebroadband cholesteric liquid crystal film adjacent to the top surfacehas a helical pitch P4, a part of the broadband cholesteric liquidcrystal film adjacent to the bottom surface has a helical pitch P5, anda part of the broadband cholesteric liquid crystal film having theshortest helical pitch P6 is located in the second region, wherein thebroadband cholesteric liquid crystal film has a first helical pitchgradient, which generally reduces from the helical pitch P4 to theshortest helical pitch P6, and a second helical pitch gradient, whichgenerally increases from the shortest helical pitch P6 to the helicalpitch P5. Further, in an embodiment of the disclosure, the distance D1between the top surface and the part of the broadband cholesteric liquidcrystal film having the shortest helical pitch P6 can be equal to thedistance D2 between the bottom surface and the part of the broadbandcholesteric liquid crystal film having the shortest helical pitch P6.Moreover, in another embodiment of the disclosure, the distance D1between the top surface and the part of the broadband cholesteric liquidcrystal film having the shortest helical pitch P6 can be different fromthe distance D2 between the bottom surface and the part of the broadbandcholesteric liquid crystal film having the shortest helical pitch P6.

Further, the thickness of the broadband cholesteric liquid crystal filmis T, the thickness of the first region is 0.4 T-0.5 T, the thickness ofthe second region is 0.2 T-0.3 T, and the thickness of the third regionis 0.2 T-0.3 T.

The method for fabricating a broadband cholesteric liquid crystal filmof this disclosure includes step (A)-(D).

In step (A), a polymerizable cholesteric liquid crystal composition anda photoinitiator are dissolved in a solvent to prepare a solution forcoating. In step (B), the solution is coated onto a substrate (such as apre-treated alignment substrate), and a CLC coating layer is obtained.In step (C), the CLC coating layer is heated in a first atmospherecontaining a first pre-fixed oxygen concentration, wherein the firstpre-fixed oxygen concentration is a fixed value of between 10-30 vol %,preferably 15-25 vol %. In step (D), the heated CLC layer is cured in asecond atmosphere containing a second pre-fixed oxygen concentration,obtaining the aforementioned broadband cholesteric liquid crystal film,wherein the second pre-fixed oxygen concentration is a fixed value ofbetween 0.01-10 vol %.

In the method for fabricating the broadband cholesteric liquid crystalfilm of this disclosure, the polymerizable cholesteric liquid crystalcomposition is not limited and can be convenient polymerizablecholesteric liquid crystal composition (including the composition madeof nematic liquid crystal and chiral compound). The solvent can beconventional single- or co-solvent for dissolving polymerizablecholesteric liquid crystal composition mentioned above, such as toluene,cyclopentanone, dimethylbenzene, cyclohexanone, propylene glycol methylether acetate (PGMEA), acetone, 2-butanone, ester, or combinationsthereof. The substrate is not limited and can be, for example, a glassor a plastic substrate applied with an alignment treatment, wherein thealignment treatment includes forming an alignment film on the substrateor rubbing directly on these substrate. Further, the CLC coating layercan be cured by irradiation by UV radiation or heat. The second oxygenconcentration of the second atmosphere is a fixed value, for example, ofbetween 0.01-10 vol %. In an embodiment, the second oxygen concentrationof the second atmosphere can be a fixed value of between 2-8 vol %. Itshould be noted that the reflection bandwidth of the broadbandcholesteric liquid crystal film can be modified by the prescribed oxygenconcentration of the atmosphere. Further, if the CLC coating layer iscured in the second atmosphere containing the second oxygenconcentration of more than 10 vol %, the obtained cholesteric liquidcrystal film would exhibit unstable and undesired film characteristics.

A method for fabricating a broadband cholesteric liquid crystal filmincludes the steps of (A)-(D): (A) a polymerizable cholesteric liquidcrystal composition and a photoinitiator are dissolved in a solvent toprepare a solution for coating; (B) the solution is coated onto asubstrate (such as a pre-treated alignment substrate), and a CLC coatinglayer is obtained; (C) the CLC coating layer is heated in a firstatmosphere containing a first pre-fixed oxygen concentration, whereinthe first pre-fixed oxygen concentration is a fixed value of between10-30 vol %, preferably 15-25 vol %; and (D) the heated CLC layer iscured in a second atmosphere containing a second pre-fixed oxygenconcentration, obtaining the aforementioned broadband cholesteric liquidcrystal film, wherein the second pre-fixed oxygen concentration is afixed value of between 0.01-10 vol %. Accordingly, in comparison withthe conventional fabricating method with long-term low-energy exposureor two-stage polymerization, the method for fabricating the broadbandcholesteric liquid crystal film of this disclosure has advantages ofhigher throughput, stable film characteristics, and reduced equipmentrequirements.

The following examples are intended to illustrate the disclosure morefully without limiting the scope of the disclosure, since numerousmodifications and variations will be apparent to those skilled in thisart.

EXAMPLE 1

2.829 g of a polymerizable nematic liquid crystal (sold and manufacturedby BASF with the trade name of LC-1057) was mixed with 0.171 g of adextrorotatory chiral compound (sold and manufactured by BASF with thetrade name of LC-756). The mixture was dissolved by 12 g of a co-solvent(toluene:cyclopentanone=4:1), then a solution for coating is obtained.Next, the solution was spin-coated on an alignment pre-treated PETsubstrate and then baked to remove the solvent. The CLC layer was thenheated under air atmosphere (having an oxygen concentration fixed at 20vol %) at 100-110° C. and cooled to ambient temperature. Finally, a 5-μmthick CLC film (A) was obtained by heating to 100-110°C. again and curedby irradiation by UV radiation in an atmosphere of an oxygenconcentration fixed at 5 vol % (mixed with nitrogen). The reflectionbandwidth of the broadband cholesteric liquid crystal film (A) wasmeasured, and the range of the reflection bandwidth was between 445-690nm.

FIG. 4 shows a scanning electron microscope (SEM) photograph of thebroadband cholesteric liquid crystal film (A) derived into three regionsaccording to the helical pitch. After measuring, the first region 102(such as the central portion of the first region) has an average helicalpitch P1 of about 368 nm, the second region 104 such as the centralportion of the second region) has an average helical pitch P2 of about275 nm, and the third region 106 (such as the central portion of thethird region) has an average helical pitch P3 of about 350 nm.Therefore, the average helical pitch P1 of the first region 102, theaverage helical pitch P2 of the second region 104, and the averagehelical pitch P3 of the third region 106 are defined by the followingequation: P1>P3>P2.

EXAMPLE 2

The same CLC solution was spin-coated on a PET substrate of alignmentpre-treated. then baked to remove the solvent. The CLC layer was thenheated under air atmosphere (having an oxygen concentration fixed at 20vol %) at 100-110° C. and cooled to ambient temperature. Finally, a 6-μmthick CLC film (A) was obtained by heating to 100-110° C. again andcured by irradiation by UV radiation in an atmosphere of an oxygenconcentration fixed at 6 vol % (mixed with nitrogen), obtaining abroadband cholesteric liquid crystal film (B). Finally, the reflectionbandwidth of the broadband cholesteric liquid crystal film (B) wasmeasured, and the range of the reflection bandwidth was between 420-700nm.

Polarization Device

According to an embodiment, referring to FIG. 5, we also provides apolarization device 200 including a broadband cholesteric liquid crystalfilm 100 (disposed on a substrate 50) and a retardation film (Z-plate,Teijin Chemical)110, wherein the retardation film 110 is disposed on thecholesteric liquid crystal film 100. The retardation film have anin-plane retardation (R_(o)) is 138 nm (measured at wavelength 550 nm)and its out-of-plane retardation (R_(th)) is about 240 nm (measured atwavelength 550 nm) , wherein the ratio |(R_(th)/R_(o))| between theout-of-plane retardation and the in-plane retardation is about 1.74.

The polarization device 200 can further include an adhesive layer 120disposed between the retardation film and the broadband cholestericliquid crystal film, wherein the adhesive layer 120 can have arefractivity of between 1.45-1.75 in order to reduce optical negativeeffects of the polarization device.

The adhesive layer 120 can be, for example, an ultraviolet curingadhesive, and can further include 0.01-10 wt % of filler (such as 0.01-6wt % of filler). The filler can include polymethyl methacrylate (PMMA),polystyrene (PS), or silicon oxide. The filler can have a diameter ofbetween 3-10 μm causing Mie scattering.

According to another embodiment, in order to obtain a high lightefficiency polarization device, an absorbing polarizer film can befurther disposed on the retardation film 110 of the polarization device200 as shown in FIG. 5.

Liquid Crystal Display

According to some embodiments, this disclosure also provides a highlight efficiency liquid crystal display including a top substrate and abottom substrate. A first electrode formed on a bottom surface of thetop substrate, and a second electrode formed on the top surface of thebottom substrate, wherein a liquid crystal cell is sandwiched betweenthe top substrate and the bottom substrate. The aforementioned broadbandcholesteric liquid crystal film is disposed inside or outside of theliquid crystal cell. Further, the liquid crystal display can furtherinclude a retardation film disposed on the broadband cholesteric liquidcrystal film. A first polarizer film and a second polarizer film can berespectively disposed on the top surface and the bottom surface of theliquid crystal cell, wherein the broadband cholesteric liquid crystalfilm is disposed outside of the liquid crystal cell. Moreover, theliquid crystal display can further include a backlight module.

Accordingly, the broadband cholesteric liquid crystal film of thisdisclosure is a single-layer film, thereby eliminating complicatedmanufacturing processes and coating defects caused when forming amultilayer cholesteric liquid crystal film. In comparison withconventional single-layer cholesteric liquid crystal films, since thecholesteric liquid crystal film of this disclosure includes threeregions with different helical pitches while maintaining a predeterminedhelical pitch distribution, the cholesteric liquid crystal film of thedisclosure has a broader reflection bandwidth. Further, due to theone-step polymerization method, the method for fabricating the broadbandcholesteric liquid crystal film of the disclosure has advantages ofhigher throughput, stable film characteristics, and reduced equipmentrequirements.

While the disclosure has been described by way of example and in termsof the preferred embodiments, it is to be understood that the disclosureis not limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

What is claimed is:
 1. A method for fabricating a broadband cholestericliquid crystal film, comprising the steps of (A)-(D): (A) dissolving acholesteric liquid crystal composition and a photoinitiator in a solventto prepare a solution; (B) coating the solution to a substrate,obtaining a cholesteric liquid crystal coating layer; (C) heating thecholesteric liquid crystal coating layer in a first atmospherecontaining a first pre-fixed oxygen concentration, wherein the firstpre-fixed oxygen concentration is a fixed value of between 10-30 vol %;and (D) curing the cholesteric liquid crystal coating layer in a secondatmosphere containing a second pre-fixed oxygen concentration, obtainingthe aforementioned broadband cholesteric liquid crystal film, whereinthe second pre-fixed oxygen concentration is a fixed value of between0.01-10 vol %.
 2. The method as claimed in claim 1, wherein thecholesteric liquid crystal coating layer is cured by irradiation by UVradiation or heat.
 3. The method as claimed in claim 1, wherein thefirst pre-fixed oxygen concentration of the first atmosphere is a fixedvalue of between 15-25 vol %.